1. Field of the Invention
The present invention relates to a developing apparatus for use for example in electrophotographic machines or electrostatic copying machines. More particularly, the invention relates to a developing apparatus including a developer roller having a conductive elastic surface. The developer roller is adapted to develop an electrostatic latent image on a photoreceptor drum by supplying a developer to the surface of the photoreceptor drum from the developer roller with the developer roller being rotated in contact with the photoreceptor drum.
2. Description of the Related Arts
Conventionally, developing apparatuses which employ a single component developer containing no carrier are simple in construction, so that size reduction, cost reduction and maintenance minimization thereof can readily be achieved. Particularly, developing apparatuses which do not employ a magnetic developer but a nonmagnetic single component toner and hence require no magnetic roller can readily be introduced for commercial use as compact and less expensive developing apparatuses which ensure formation of clear images.
The developing apparatuses employing such a nonmagnetic single component toner are classified into two broad categories: a noncontact type developing apparatuses in which the image development is achieved by applying an alternating electric field between a photoreceptor drum and a developer roller spaced a predetermined distance from the photoreceptor drum to cause the toner to fly back and forth between the photoreceptor drum and the developer roller; and contact type developing apparatuses in which the image development is achieved by bringing a conductive elastic developer roller into contact with a photoreceptor drum.
The noncontact type developing apparatuses require a complicated power supply system capable of withstanding a higher voltage for applying thereto an AC-superposed DC developing bias. On the other hand, the contact type developing apparatuses simply require an DC power supply for applying thereto an DC developing bias and, therefore, are advantageous in that the power supply system is simplified without a requirement for a higher breakdown voltage.
The contact type developing apparatuses are advantageous in many aspects as described above. However, the contact pressure and nip width for keeping the developer roller in contact with the photoreceptor drum greatly influence the quality of an image to be formed.
Japanese Unexamined Patent Publication No. SHO 59(1984)-223469, for example, proposes a contact type developing apparatus capable of keeping the contact pressure and the nip width constant. The contact type developing apparatus includes a pressing member for biasing a conductive elastic developer roller against the surface of a photoreceptor drum and a tracking roll for controlling a biasing pressure to a predetermined level. Further, Japanese Unexamined Patent Publication No. SHO 63(1988)-155067 discloses a developing apparatus which includes a developer roller having an elastic member and a rigid restricting roller fitted to each end of the developer roller and having a smaller diameter than the developer roller.
An explanation will be given to adjustment of the contact depth of the developer roller pressed against the photoreceptor drum by the tracking roll or the restricting roller with reference to FIGS. 6 and 7.
FIG. 6 is a sectional view illustrating a relationship between a photoreceptor drum and a contact depth controlling roller in a conventional contact type developing apparatus as seen from a lateral side. FIG. 7 is a sectional view illustrating the relationship between the photoreceptor drum and the contact depth controlling roller as seen from the front side. In the contact type developing apparatus shown in FIGS. 6 and 7, a developer roller 22 having an elastic surface is kept in contact with the photoreceptor drum 21, and the rigid contact depth controlling roller 25 (tracking roll or restricting roller) which has a diameter smaller by a predetermined amount than the developer roller 22 is rotatably engaged with each end of a shaft 23 of the developer roller 22 via a ball bearing 24. Thus, the contact depth of the developer roller 22 is mechanically kept constant on the basis of a difference in diameter between the developer roller 22 and the contact depth controlling roller 25.
However, the conventional contact type developing apparatus should be provided with a contact depth controlling roller comprised of a ball bearing or a sleeve bearing for accommodating a difference in circumferential velocity between the photoreceptor drum and the developer roller to ensure stable image development while maintaining a predetermined image density.
In addition, the conventional contact type developing apparatus having such a complicated contact depth controlling mechanism suffers fluctuation in the contact nip width and an increase in load torque exerted on a developer roller driving mechanism, which may result from wear-out of sliding contact surfaces and fusing of toner on the bearing and the sliding contact surfaces.